The present invention relates to an anisotropicelectroconductive adhesive film which is electrically conductive only in the direction perpendicular to the surface thereof and not in any direction parallel to the surface.
Solders and electroconductive adhesives have so far been used widely as means of connecting large scale integrated circuits (LSI) or electrodes of liquid crystal display elements (LCD) to printed circuit boards and of bonding flexible flat cables (FFC) to external connecting terminals of various devices.
However, with the miniaturization of electronic parts and the size reduction of connecting circuits, it has become extremely difficult to connect circuits to one another without causing short circuit between neighboring parts of the circuits in particular when connecting terminals or the like are aligned at fine pitches.
In addition, soldering has a drawback of low reliability in adhesion due to hard and brittle properties thereof, that is, the liability of bonded parts to be readily separated by a shock or the like.
Electroconductive adhesives, on the other hand, are expensive because electroconductive fillers need to be incorporated to high concentrations of at least 20% by volume into adhesives in order to obtain necessary conductivity. Further, drawbacks of conductive adhesives are that variation in the conductivity is caused by such a factor as the precipitation of the filler during storage or difficulty in uniform coating thereof and that environmental pollution due to the contained organic solvent may occur.
An example of the attempts to eliminate these drawbacks is the method disclosed in Japanese Patent Application Kokai (Laid-Open) No. 147,732/78, which comprises applying a conductive adhesive prepared by incorporating 5 to 25% by weight of an electroconductive carbon black as an electroconductive filler into a heat-sensitive adhesive and conducting an electric current through the layer of the applied adhesive to generate Joule's heat, thereby completing bonding.
According to this method, however, the upper limit of the electroconductivity is 10.degree. .OMEGA.-cm.sup.-1, the appearance is black in color, and the transparency is lost completely, since carbon is used as the electroconductive filler. Moreover, the bonding operation requires a long time ranging from 10 minutes to 10 hours and the workability is inferior, since the adhesion is achieved with Joule's heat.
Adhesive tapes having electroconductive anisotropy are also known which are prepared by dispersing metal particles as electroconductive filler particles in a polymer binder (Japanese Patent Application Kokai (Laid-Open) No. 101040/76). Such adhesive tapes, however, are inferior in adhesive properties and transparency, since conductive fillers such as metal particles need to be incorporated in large quantities of at least 10% by volume in order to secure the necessary electroconductivity.
Further, for circuit connecting purposes, there are used in practice connecting materials having anisotropic-electroconductivity which materials are produced by linear alignment of metal wires, carbon filaments, or carbon particles in polymers, for example, silicone rubber. However, these connecting materials have a number of drawbacks in that the minimum separation width of the conductors or insulators is as large as 0.2 mm at best, thus the use of these materials for connecting fine circuits is limited, compression pressure should always be applied in order to maintain the connection in a good state and this requires a special fixing means, which results in restricting equipment designs; and the materials themselves are opaque so that the adjustment of relative positions of circuits is complicated when the circuits are superposed and connected together.